The present invention relates to copy protection systems and in particular to a technique and associated implementations which enable the recovery and reproduction of a tag signal in a watermarked and/or copy protected video signal.
The tag signal is added to the video signal (i.e., program material) explicitly and carries an information content that relates to the content of the invisible watermark signal on that same video signal. The tag signal is time-varying and of fixed duration, and simply repeats for the length of the protected program material. The tag signal is designed to be non-reproducible by selected conventional recording/reproducing means such as consumer digital VCRs, whereas the watermark is designed to be carried along in the active video through any and all processes. Compliant recorder/reproducers, detecting the absence of the tag signal on watermarked material, will interpret the absence as indicating that the program is indeed a copy, and thereby refuse to record or reproduce the signal. Such a tag/watermark system is intended to prevent the production of unauthorized second generation copies as well as the playback of illicit second generation copies.
By way of example, a xe2x80x9ctagxe2x80x9d (that is, a tag signal) may comprise a small burst of color inserted in a corner of the active video picture. The burst of color is too small and too finely detailed to be reproduced by compliant recorder/reproducers. That is, the tag is a high frequency signal that, in this example, is outside the bandwidth of the low frequency bandwidth chroma channel of the recorder/reproducers.
In another example, a tag may comprise a series of pulses inserted outside of the active video picture in the blanking interval. Because of well known bandwidth limitations, digital recorders conserve bandwidth by not recording blanking intervals. The timing information provided by the blanking intervals is not required in digital recorders since the video signal is stored digitally. When digital reproducers play back the video signal, they simply insert a stored blanking interval in the proper location. It follows therefore, that a tag inserted in the blanking interval is discarded along with the blanking interval during a recording process. When the digital recorder/reproducer reconstructs the video signal and blanking intervals on playback, the tag is not available and thus is not reproduced.
By way of example only, a typical tag signal used in a generational copy control system may be found in co-pending U.S. Provisional Application Serial No. 60/088,682 filed Nov. 5, 1999 entitled xe2x80x9cGenerational Copy Control of a Digital Video Signal.xe2x80x9d
The present invention comprises a method and apparatus by which the tag signal in copy protected program material, that is, video signals, may be detected, stored and then added back to the generally tagless output of the copy, thereby enabling the production of further (second generation and beyond) playable copies of the protected program.
To this end, the tag signal is recovered from the copy protected program material, that is, read, by conventional means. As an example, if the tag signal is a pulse or series of pulses, such as the aforementioned burst of color in the picture or the pulse outside the picture, at a specific location or locations in the video signal waveform, appropriate timing means may be provided to select the desired location(s). At these locations a pulse-slicer may be used to convert the tagged video waveform into a conventional logic-level signal.
In accordance with an embodiment of the present invention, herein termed an inframe tag embedded technique, the recovered tag signal (which was located in the blanking interval) is converted into an in-frame video component (that is, a xe2x80x9ctag representationxe2x80x9d) by any suitable means. The in-frame component is then added to the program video signal from the original source and the resultant composite signal is recorded conventionally by a generally compliant first generation recorder which may, or may not, strip off the original tag, while the added in-frame component is recorded as part of the picture information. Upon playback of the first generation copy, the added in-frame component is detected and a regenerated tag waveform conforming to the original specification is synthesized in real time. The regenerated tag waveform is added with the playback video which is then recorded conventionally on a generally compliant second generation recorder to produce a second generation copy which is fully playable.
In accordance with an alternative embodiment of the invention, herein termed a parallel recorder/reproducer technique, the recovered tag signal is converted to a video signal by any appropriate means, for instance by amplitude modulation, and is recorded as a xe2x80x9ctag representationxe2x80x9d on a separate recording means for the entire length of the program material, concurrently and synchronously with the recording of the program material on the primary recorder. Upon playback, the two recorder/reproducers are again synchronized and the tag information in the output of the separate recording means is recovered, and from that information the original tag signal waveform is synthesized in real time at the appropriate instants. The synthesized tag signal waveform is then added in real time to the program material which is then suitable for input to a compliant second generation recorder for playback.
In accordance with another alternative embodiment of the invention, herein termed a computer storage technique, a pulse stream represented by the recovered tag signal, at one pulse per video field, is converted into a digital sequence, that is, a xe2x80x9ctag representation,xe2x80x9d for the duration of the program material. The complete sequence of tag information is then stored in a computer file in any convenient form, for example as a straight binary file, etc. The second generation copy is then made by playing back the first generation copy, and simultaneously reading the previously written file in order to synthesize the regenerated tag waveform in real time. The tag waveform is then added to the tagless video signal played back from the first generation copy prior to supplying the combined signal to the compliant second generation recorder.
In accordance with another alternative embodiment of the invention, herein termed a correlation technique, since the exemplary tag bit sequence used in the previous computer storage embodiment repeats, it is not necessary to store the sequence for the whole program material. It suffices to store only one cycle thereof. It is, however, necessary to determine the exact length of the actual tag bit sequence, in order to repeat it properly. This is most easily done by storing a number of bits well in excess of any reasonable tag sequence length, such that the stored sequence contains at least one complete digital tag length. Conventional autocorrelation techniques are then used to determine the actual digital tag length, and one sequence""s worth of tag bits are then stored in a computer file. The second generation copy is then made by playing back the first generation copy, and simultaneously and repetitively reading the previously-written file in order to synthesize the requisite tag waveform in real time. Then, the tag waveform is added to the tagless video from the first generation copy before it is fed to the compliant second generation recorder.
In accordance with still another alternative embodiment of the invention, termed a hardware correlation technique similar to the computer storage and correlation embodiments, since the tag bit sequence repeats, again it is not necessary to maintain the sequence for the whole length of the program material. It suffices to determine only one cycle thereof while determining the exact length of the actual digital tag sequence in order to repeat it properly. This is achieved in this embodiment by a hardware autocorrelator. The digital sequence from the tag recovery unit is applied to a binary shift register of length adequate to store a pulse sequence well in excess of the expected tag signal length. The shift register is supplied with a matching array of logical exclusive-or (xe2x80x9cXORxe2x80x9d) gates. The output of each stage of the shift register is applied to one input of the associated array of XOR gates. The digital sequence from the tag recovery unit is also directly applied in parallel to the other input of all of the XOR gates in the array. The output of the XOR associated with the stage in the shift register corresponding to the length of the tag bit sequence will produce constant logical xe2x80x9c1""sxe2x80x9d, since the direct input will exactly match the delayed input. The output from each XOR gate is collected in an associated accumulator and the accumulator with the highest value indicates the length of the tag bit sequence.
Once the length of the tag bit sequence is determined, that many consecutive bits from the input tag bit stream are stored locally. The second generation copy is then made as described in the immediately previous embodiment.